1. Field of the Invention
The invention relates to the preparation and processing of thermoplastic compositions based on vinyl chloride polymers with excellent notched impact strengths. The modification for notched impact strength uses a shell-type graft copolymer which is low in rubber and is added as a polymer latex prior to or during the polymerization of the vinyl chloride, whereupon the polyvinyl chloride (PVC) produced grafts onto the modifier latex particles. The component which improves impact strength is composed of a hard core and of a soft, rubber-like shell.
2. Discussion of the Background
The good price-performance ratio of polyvinyl chloride (PVC) and its versatility in use make it one of the most widely used polymers. However, PVC on its own is too brittle for many applications, e.g. window profiles. To improve the impact strength of PVC, vinyl chloride polymers have in the past been provided with a wide variety of modifiers. Examples of these which may be mentioned are polymeric impact modifiers of butadiene type, such as acrylonitrile-butadiene-styrene (ABS) and methyl methacrylatebutadiene-styrene (MBS); copolymers of ethylene with vinyl acetate (EVA); chlorinated polyolefins, such as chlorinated polyethylene (CPE); ethylenepropylene rubbers and polymers of acrylate type, such as homo- and copolymers of alkyl acrylates. The application DE 1,082,734, for example, describes a process for preparing impact-modified polyvinyl chloride. The polymer claimed is prepared by polymerizing vinyl chloride in aqueous suspension with the aid of suspension stabilizers and of organic or, respectively, inorganic activators, and the polymerization of the vinyl chloride takes place in the presence of aqueous emulsions, of polymers which have tough and resilient properties at room temperature and are present in amounts of from 2 to 25% by weight, based on solids. The polymers here may be homopolymers of acrylic or vinylic esters or, respectively, copolymers with other compounds.
A disadvantage of this process is that to produce profiles a very large amount of the expensive acrylate is required to achieve sufficiently high notched impact strength in, for example, a PVC profile.
Grafted or core-shell impact modifiers with a layer-like structure are also known in principle. DE 4,302,552 describes graft and core-shell copolymers with improved phase compatibility between graft base and the polymer phase grafted on. The graft and core-shell copolymers are prepared from a polymer phase a) containing peroxy groups and comprising from 0.01 to 20% by weight of a doubly olefinically unsaturated peroxy compound of the formula H.sub.2 C.dbd.CH--O--CO--R.sup.1 --CO--O--O--CO--R.sup.1 --COO--CH.dbd.CH.sub.2 and contains from 80 to 99.99% by weight of one or more comonomers selected from the group consisting of (meth)acrylates of alcohols having from 1 to 10 carbon atoms, vinyl esters of saturated aliphatic carboxylic acids having from 2 to 10 carbon atoms, olefins, vinylaromatics, vinyl halides and/or vinyl ethers, and from, grafted onto this, a polymer phase b) which is prepared by grafting one or more comonomers selected from the group consisting of (meth)acrylates of alcohols having from 1 to 10 carbon atoms, vinyl esters of saturated aliphatic carboxylic acids having from 2 to 10 carbon atoms, olefins, vinylaromatics, vinyl halides and styrene, and also styrene derivatives, onto the polymer phase a) containing peroxy groups. A disadvantage of this process for preparing core-shell polymers is that concomitant use of an unstable comonomer containing peroxy groups is required in order to ensure phase compatibility between polymer phases a) and b), and care has to be taken that the peroxide functions are not destroyed. The text also describes the use as an impact modifier in plastics, albeit in solid form. This, however, is another disadvantage since it necessitates an additional work-up process, namely drying. The shell is moreover used in uncrosslinked form, and this results in some shearing away of the shell polymer during processing and is highly disadvantageous.
EP 0,600,478 also describes the preparation of a graft copolymer latex from core-shell dispersion particles with improved phase compatibility between core and shell, using a two-stage emulsion polymerization process. However, only crosslinked, elastomeric polymers are permissible in the first stage. In addition, the shell polymer has to have a glass transition temperature (T.sub.g) above 20.degree. C., and this would have an adverse effect for the use as impact modifier in thermoplastics.
There are also known core-shell modifiers for improving the notched impact strength of PVC, which have a hard core and a soft shell made from rubber-like material. For example U.S. Pat. No. 3,763,279 and DE 3,539,414 describe the preparation of polymer systems which have a hard, crosslinked core made from polystyrene and a soft, crosslinked polyacrylate shell. Disadvantages are firstly the relatively poor compatibility of the polystyrene core with the PVC matrix, the effect of which is especially adverse when welding PVC profiles which have been cut to the required dimensions. Secondly, these processes were optimized for transparency, and polystyrene therefore had to be used as core material. This is uneconomic, however, when transparency is not needed in the resultant semifinished product.